Watch them regularly, is their habits are changing? Working out, dressing, using strong seductive perfumes.

You can check their call logs when phone is on charging, look for call or massages which randomly showing lot of time in their log. They usually save their number with name co-worker or friend

Tell them you are going out of station and stay there to check them, give surprise visit to their office, also working for late in office and then come earlier.

You can use voice recorder, hidden cameras also you can download software in their computer to track their activities; you can use GPS tracking devices to find out their location. And if this all not works then you can hire a private investigator too.